The invention relates to a vacuum cleaner, particularly but not exclusively to a dual cyclonic vacuum cleaner.
A dual cyclonic vacuum cleaner comprises a dirty air inlet communicating with a clean air outlet by means of an airflow path, two cyclones being sequentially arranged in the airflow path. In use, air flowing along the airflow path from the dirty air inlet to the clean air outlet passes through a first of the two cyclones and subsequently through a second of the two cyclones. The first cyclone is a "low efficiency" cyclone designed to remove relatively large particles from the airflow, whilst the second, "high efficiency" cyclone is designed to remove fine dust particles from the airflow. A vacuum cleaner having these features expels air which is dirt- and dust-free to a higher degree than other known vacuum cleaners. Examples of such vacuum cleaners are known from published European application No. 0489565 and European patents Nos. 0042723 and 0134654.
Another advantage of the dual cyclonic vacuum cleaner is that the dirt-collecting chambers are highly unlikely to become blocked because of the size and rigidity of the chambers. However, it is inevitable that the dirty air inlet, either in the form of a cleaner head or a tool attached to a hose or wand, can become blocked to a greater or lesser extent. Naturally, this reduces the airflow along the airflow path. A single cyclonic vacuum cleaner operates in the same manner but utilises only one cyclone which can become inefficient if the airflow rate though the cyclone is reduced.
Vacuum cleaner airflow rates are measured at various orifice sizes. The flow rates start at an effective orifice size of 50 mm diameter and are reduced to zero at zero diameter. Any flow rate in any given machine therefore has an equivalent "effective orifice" size. In practice, a vacuum cleaner being used through a hose or wand typically has an effective orifice size of 32 mm diameter if it is fully open. A vacuum cleaner operating on a carpet through a cleaner head has an effective orifice of about 19 mm diameter. A crevice tool being used on the end of a wand handle may have an effective orifice of about 15 mm diameter. Thus it can been seen that, in its normal range of use, a vacuum cleaner has to deal with airflows equivalent to those obtained through orifices of from 15 mm to 32 mm diameter.
At all of these flow rates achieved in normal use, the second cyclone of a dual cyclonic vacuum cleaner maintains a good Level of fine dust separation. However, it has been found that the separation efficiency of the second cyclone is reduced if the airflow rate through the second cyclone is reduced to below that of an effective orifice size of 13 mm. This can be caused by a number of things; for example, a blockage occurring at any point along the airflow path, or by the user putting a hand or other object over the air inlet. Furthermore, the efficiency of the second cyclone is reduced if the flow is interrupted in a pulsing manner or if the suction through the cleaner head causes the cleaner head to seal itself partially or completely against the surface to be cleaned. A similar problem arises when the airflow through the cyclone of a single cyclonic vacuum cleaner is reduced.
Depending upon the specific design of the cyclonic vacuum cleaner, the air discharged from a cyclonic vacuum cleaner may be substantially dust free and may in fact be cleaner than the air which is emitted from a vacuum cleaner which utilises a bag or other filter media. However, under certain operating conditions, cyclonic vacuum cleaners may emit larger than desired quantities of fine particulate matter. For example, if the vacuum cleaner picks up a particularly heavy concentration of fine particulate matter, part of the fine particulate matter may pass through the two cyclones and be exhausted from the second cyclone. This may result in the deposition in a room of a layer of fine dust particles. Further, the filtered exhaust air may be passed by the,, motor housing to cool the motor. If the exhaust air occasionally includes more than desired quantities of fine particulate matter, the motor may experience a build up of fine particulate matter which could decrease; the life expectancy of the motor.